In the Output panel of the Export Adobe PDF dialog, you have certain options for compressing the images in your document to help with file size. As mentioned earlier, compression can be applied to color, grayscale, and monochrome images (see Figure 31.14).
Figure 31.14. Take care when compressing images in your PDF; this can make the file smaller, but it can also degrade image quality if you print to a high-resolution output device.
Your choices include
Average Downsampling, which replaces specific pixels in the image with the average pixel color in a sample area.
Subsampling, which picks a pixel in the center of a sample area and uses it as the color for the entire area. This results in the fastest conversion time, but the least smooth look.
Bicubic Downsampling, which uses a weighted average to sample pixel color. This results in the slowest conversion time, but has the smoothest appearance.
Choose from the following compressed image formats:
Automatic (JPEG), which is the best quality for color and grayscale images. This option is available only for documents exported to Acrobat 6 and later.
JPEG, which tries to reduce file size with the least amount of lost information.
ZIP, which can be lossless or lossy, depending on whether the resolution is set at or below the image's original resolution.
JPEG 2000, which is the international standard compression format. This option enables you to set progressive display (enter the number of tiles in the field to the right) and is available only in documents set to export to Acrobat 6 or higher.
Automatic JPEG 2000, which automatically determines the best quality for image reproduction.
The Image Quality pop-up menu determines the amount of compression.
For monochromatic images, the compression options include CCITT, which is good for black-and-white images. Group 4 is a general purpose compression method; Group 3 is used by fax machines and compresses the bitmaps one row at a time. Run Length compression works well for images with large solid areas of black or white.
The Compress Text and Line Art option applies a flat compression without degrading detail or quality.
The Crop Image Data to Frames option deletes image areas that are hidden outside the frames in which they are placed.